S-(ω-hydroxyalkyl) esters of thioacrylic and thiomethacrylic acids

ABSTRACT

Compounds are described which are S-(ω-hydroxyalkyl) esters of thioacrylic and thiomethacrylic acids. The compounds have the formula: ##STR1## wherein: R is hydrogen or methyl; and 
     n is 1, 2 or 3. 
     These compounds can be used as conditioners to enhance the wettability of dentinal surfaces, can be employed as chemical tracers to characterize the wetting of dentinal surfaces, and can be copolymerized in situ to form potential dentinal bonding systems. The compounds can also be homopolymerized and copolymerized to form other useful products.

BACKGROUND OF THE INVENTION

The government has certain rights in this invention pursuant to ResearchGrant No. NIDR DE 09696.

This invention relates in general to compositions of matter and, moreparticularly, to thiomethacrylic and thioacrylic acid ester compounds,methods of preparation of such compounds and polymers made therefrom.

Dental composites are typically applied in a multi-step process designedto provide a secure adhesive bond and seal at the interface of thedental adhesive and tooth surface. As an example, when the restorativeis a cavity filling, the decayed tooth material is cut away and anetching composition such as citric acid is applied to the tooth surface.The etching composition serves to remove debris remaining from thecutting process and causes demineralization of a thin layer ofunderlying dentin to facilitate the wetting and penetration of thesubsequently applied dental adhesive. A conditioner such as2-hydroxyethylmethacrylate (HEMA) is also commonly applied to the dentinprior to or concurrently with application of the dental adhesive toenhance the wetting, penetration and bonding of the dental adhesive tothe dentin.

The dental adhesive must be capable of wetting the prepared dentinsurface as well as bonding with the organic composite resin which iscommonly used to fill the hollow area in the portion of the tooth thathas been cut away. The dental adhesive must also possess a sufficientshear bond strength to withstand the shrinkage experienced by manydental composite resins, such as those based on bis-GMA(2,2'-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)]phenyl propane). Theadhesive is typically a vinyl monomer mixture which undergoes chemicalor photo-initiated polymerization. The composite resin may include thesame or different monomer material as used for the dental adhesive, aswell as suitable filler material.

HEMA homopolymers and copolymers have also been used extensively asbiomaterials in diagnostic and therapeutic devices and for implants.These polymer materials are known as hydrogels because of theirhydrophilicity and insolubility in water. The high water content, softand rubbery consistency and low interfacial tension of HEMA polymersclosely resemble those same properties found in living tissues. Becauseof these similarities, HEMA polymers have been used as diagnosticdevices such as catheters, electrode catheters, carriers for enzymeimmunoassay, gel-entrapped enzyme electrode probes, cell culturesubstrates, and electrophoresis gels. HEMA polymers have also been usedin therapeutic applications as absorbent coatings for blood perfusion,hemodialysis membranes, blood oxygenators, degradable therapeuticsystems, drug-delivery systems, and medicated and soft contact lenses.Implant uses of HEMA polymers includes intraocular lenses, artificialcorneas, soft tissue substitutes, burn dressings, transdermaldrug-delivery patches. Other uses are known, including as separationmembranes.

Despite these widely varying uses of HEMA polymers, it would bedesirable in certain applications to achieve a faster rate of vinylpolymerization of the HEMA monomer. Another problem presented by the useof HEMA as a dental restorative conditioner and in other tissueapplications is the difficulty in determining the degree of penetrationof HEMA into the underlying dentin or tissue. Radioactive labellingprocedures can be used to determine the degree of penetration of HEMA,but it would be desirable to use alternate procedures which do notrequire the use of radioactive markers. For example, compoundscontaining sulfur atoms can be localized in tissue samples usingenergy-dispersive spectroscopy and auger spectroscopy.

Alkyl thiolacrylates are known and methods of preparation have beenreported. Although alkyl thiol esters have been obtained by thetreatment of a mercaptan with an acid chloride, the use of acryloylchloride in this process has been generally unsuitable because themercaptan is added to the carbon-carbon double bond at the same time asthe formation of the ester. Instead of directly forming the thiolesters, Marvel et al. in Journal of Polymer Science, Vol. XIX, pages59-71 (1956), described the process of preparing saturated alkylthioesters by first reacting mercaptan with α,β-dibromopropionylchloride to form the thiol ester, followed by the elimination of thebromine atoms by the addition of sodium iodide.

Braude, in J. Organic Chem., Vol. 22, pages 1675-78 (1957), reportedthat the thioesters could be prepared by contacting methacryloylchloride with the lead salt of the mercaptan. Sumrell et al. laterdisclosed that methacryloyl chloride could be directly reacted withmethyl mercaptan in the presence of sodium hydroxide at a temperaturebelow 10° C. to form methyl thiolmethacrylate. J. Amer. Chem. Soc., Vol.80, pages 2509-13 (1958). Otsu et al. have also reported on thecopolymerization of several alkyl thioacrylates with styrene in DieMakromolekulare Chemie, Vol. 119, pages 140-146 (1968).

SUMMARY OF THE INVENTION

It is an object of this invention to provide a compound which hasproperties comparable to HEMAbut contains a thiol group which can belocalized using energy-dispersive spectroscopy so that the degree ofwetting and penetration of the compound into the substrate to which itis applied can be determined without the use of radioactive labellingprocedures.

It is also an object of this invention to provide a compound which canbe polymerized to form homopolymer or copolymer materials for use in awide variety of biomaterial applications.

As a corollary to the preceding object, it is another object of thisinvention to provide a compound which has a faster rate of vinylpolymerization in comparison to HEMA so that the compound can be used inplace of HEMA, including in those applications where the faster rate ofpolymerization is more suitable.

In one aspect, the invention provides novel compounds of the generalformula I: ##STR2## wherein: R is hydrogen or methyl; and

n is 1, 2 or 3.

In another aspect, the invention is directed to a method of preparingcompounds of formula I, said method comprising serially carrying out thefollowing steps:

(a) contacting an ω-mercaptoalkanol with an oxy-blocking agent toprepare an oxy-blocked-ω-mercaptoalkanol;

(b) contacting the oxy-blocked-ω-mercaptoalkanol with a suitablemetallic substance to prepare a metallic mercaptide-salt of theoxy-blocked-ω-mercaptoalkanol;

(c) contacting the metallic mercaptide-salt of theoxy-blocked-ω-mercaptoalkanol with an acryloyl or methacryloyl halide orpseudo halide to prepare an oxy-blocked-S-(ω-hydroxyalkyl) ester of athioacrylic or thiomethacrylic type acid; and

(d) contacting the oxy-blocked-S-(ω-hydroxyalkyl) ester of a thioacrylicor thiomethacrylic type acid with a suitable deprotecting agent toprepare the S-(ω-hydroxyalkyl) ester of a thioacrylic or thiomethacrylictype acid.

The invention is also directed to homopolymers and copolymers whichcomprise a polymer containing a residue of a compound of the foregoingformula I and/or which have a repeating unit of the following generalformula: ##STR3## wherein R and n are as defined above.

In another aspect, the invention is directed to compounds of the generalformulas II and III: ##STR4## wherein: R and n are as defined above;

M is a metal residue having a valence of m;

m is generally from 1 to 4;

Y is an oxy-blocking agent residue; and

y is the valence of Y, often equal to m, e.g., 1 or 2. The compounds offormulas II and III are useful intermediates in the preparation ofcompounds of formula I.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention is directed, in part, to S-(ω-hydroxyalkyl) esters ofthioacrylic and thiomethacrylic acids of the general formula I: ##STR5##wherein: R is hydrogen or methyl; and

n is 1, 2 or 3.

Preferred compounds of formula I are those in which R is methyl orhydrogen and n is 1 or 2. More preferred are compounds in which R ismethyl or hydrogen and n is 1.

Compounds encompassed within formula I include S-(2hydroxyethyl)thiomethacrylate, S-(2-hydroxyethyl) thioacrylate,S-(3-hydroxy-n-propyl) thiomethacrylate, S-(3-hydroxy-n-propyl)thioacrylate, S-(4-hydroxy-n-butyl) thiomethacrylate, andS-(4-hydroxy-n-butyl) thioacrylate. Particularly preferred areS-(2hydroxyethyl) thiomethacrylate and S-(2-hydroxyethyl) thioacrylate.

Unless specifically indicated to the contrary, to the extent thecompounds of the invention may exist as optical or geometric isomers,all isomers and racemic mixtures are understood to be included in theinvention. In addition, all possible isomeric forms of the compounds ofthe invention are within the ambit of this invention.

The invention is also directed to compounds of the general formulas IIand III: ##STR6## wherein: R is hydrogen or methyl;

n is 1, 2 or 3;

M is a metal residue having a valence of m;

m is generally from 1 to 4;

Y is an oxy-blocking agent residue; and

y is the valence of Y, often equal to m, e.g., 1 or 2.

Compounds of formulas II and III are useful intermediates in thepreparation of the compounds of formula I. The metal residue in theseintermediate compounds may be from any suitable metal. For instance, itmay be of lithium or lead. Preferably, the metal residue is Pb(II). Themetal may be introduced, for example, into an oxy-blockedmercaptoalkanol precursor, by employing a suitable substance (e.g., ingeneral, MX, preferably M(X)_(y) where M and y are as defined above andX is a suitable counterion, both present in suitable valence-balancedform). Preferred counterions are acetates.

The oxy-blocking agent (which may be represented by the formula YR',where Y is as defined above and R' comprises a group or part to completethe agent) is a substance which derivatives the required hydroxyl moietyof the S-(ω-hydroxyalkyl) thioacrylate and thiomethacrylate compound ofthe general formula I. This agent replaces the active hydrogen of thenoted hydroxyl moiety and generally can be removed later on as bysubstitution of another hydro moiety for the oxy-blocking agent.Preferably, the oxy-blocking agent is a silicon-containing blockingagent, which may include so-called silane blocking, or silylating,agents. Some of the more prominent silylating agents includetrimethylchlorosilane, hexamethyldisilazane, trimethylsilylamides,trimethylsilylureas, trimethylsilylamines, trimethylsilyl sulfates,t-butyldimethyl chlorosilane, t-butyldiphenylchlorosilane, andtriisopropyl chlorosilane. General use of these particular agents isknown in the art. Of the foregoing, hexamethyldisilazane andtrimethylchlorosilane, the latter typically employed with a protonacceptor such as pyridine or triethylamine, are preferred.

Removal of the silicon-containing blocking agent can be carried out byemployment of a suitable deprotecting agent (HR", wherein R" comprises agroup or part to complete the agent which contains a donarable hydromoiety (H)). The deprotecting agent is a substance which can remove theresidue of the silicon-containing blocking agent and substitute ahydrogen back onto the protected oxy (--O--) moiety, for example, byacidic or basic hydrolysis employing a suitable alcohol such as, forexample, methanol, ethanol, propanols, and so forth and the like.

In general, conditions are those sufficient to carry out the pertinentstep or process. Specific conditions can vary depending upon specificreagents employed and effects desired.

The following general reaction scheme can be employed to preparecompounds of the present invention:

Step 1:

    yHSCH.sub.2 (CH.sub.2).sub.n OH+YR'→[HSCH.sub.2 (CH.sub.2).sub.n O].sub.y Y+HR'

Step 2:

    [HSCH.sub.2 (CH.sub.2).sub.n O].sub.y Y+M(X).sub.y →M[SCH.sub.2 (CH.sub.2).sub.n OY].sub.m +yHX

Step 3: ##STR7## Step 4: ##STR8## wherein: R, Y, R', R", M, y, m and nare as set forth above; and

X' is Cl, Br, I and the like.

Compounds of the present invention can be prepared in accordance withthe following more detailed reaction scheme:

Step 1:

    2HSCH.sub.2 (CH.sub.2).sub.n OH+[(CH.sub.3).sub.3 Si].sub.2 NH→2HSCH.sub.2 (CH.sub.2).sub.n OSi(CH.sub.3).sub.3 +NH.sub.3

Step 2:

    2(2HSCH.sub.2 (CH.sub.2).sub.n OSi(CH.sub.3).sub.3)+Pb(OOCCH.sub.3).sub.2 →Pb[SCH.sub.2 (CH.sub.2).sub.n OSi(CH.sub.3).sub.3 ].sub.2 +2CH.sub.3 COOH

Step 3: ##STR9## Step 4: ##STR10## wherein R and n are as set forthabove.

As a specific example, when R is methyl and n is 1, the reaction schemecan be carried out in the following manner. In step 1, 2-mercaptoethanolis treated with hexamethyldisilazane to silylate the hydroxy group toprotect it from reacting with the acid chloride in subsequent reactionsteps. The mercapto group does not react with the silylating reagent. Instep 2, the reaction product from step 1,(2-mercaptoethoxy)trimethylsilane is reacted with lead (II) acetatewhich was been slurried in an inert solvent. The reaction product is thelead salt of the O-silylated mercaptoethanol. In step 3, the lead saltsuspended in diethyl ether is reacted with methacryloyl chloride to formO-silylated methacrylol thiol ester and insoluble lead chloride which isremoved by filtration. In step 4, methyl alcohol is added to thefiltrate to hydrolyze the trimethylsilyl group from the reaction productof step 3. After removal of the solvent, a stabilizer, such asmethoxyhydroquinone, is added to the residue to inhibit polymerization.The residue is subjected to fractional distillation at low pressure(0.01 mm Hg) to remove the methoxytrimethylsilane by-product. The finalproduct, S-(2-hydroxyethyl) thiomethacrylate, is obtained as a clearmobile fluid in an overall yield of 70%.

Variations in the reactants and conditions can, of course, be employed.For example, in step 1, a mixture of trimethylisilyl chloride andtriethylamine can be used as the silylating agent. In step 2, anhydrouslead (II) acetate suspended in refluxing toluene can be reacted with thesilylated mercaptoethanol to obtain the mercaptide; or lead (II) acetatetrihydrate suspended in diethyl other can be reacted directly with thesilylated mercaptoethanol, provided the temperature is maintained at 0°C. to prevent hydrolysis of the silyl group. In step 3, metallicderivatives other than lead, such as lithium, can be employed in thereaction with methacryloyl chloride. In step 4, when hydrated lead (II)acetate has been used in step 3, the addition of methanol is notnecessary for hydrolysis because the acetic acid and water (generated instep 3) are sufficient to hydrolyze the silyl group when the temperatureis increased during solvent removal.

The invention is thus directed to a process to prepare anS-(ω-hydroxyalkyl) ester of a thioacrylic or thiomethacrylic type acidcomprising serially carrying out, the following steps:

(a) contacting an ω-mercaptoalkanol with an oxy-blocking agent toprepare an oxy-blocked-ω-mercaptoalkanol;

(b) contacting the oxy-blocked-ω-mercaptoalkanol with a suitablemetallic substance to prepare a metallic mercaptide-salt of theoxy-blocked-ω-mercaptoalkanol;

(c) contacting the metallic mercaptide-salt of theoxy-blocked-ω-mercaptoalkanol with an acryloyl or methacryloyl halide orpseudo halide to prepare an oxy-blocked-S-(ω-hydroxyalkyl) ester of athioacrylic or thiomethacrylic type acid; and

(d) contacting the oxy-blocked-S-(ω-hydroxyalkyl) ester of a thioacrylicor thiomethacrylic type acid with a suitable deprotecting agent toprepare the S-(ω-hydroxyalkyl) ester of a thioacrylic or thiomethacrylictype acid.

The ω-mercaptoalkanol is of the general formula HSCH₂ (CH₂)_(n) OHwherein n is 1, 2 or 3; the oxy-blocking agent is a silicon-containingblocking agent; the metallic substance is a salt or compound of lithiumor lead; the acryloyl or methacryloyl halide or pseudo halide is achloride; and the S-(ω-hydroxyalkyl) ester of a thioacrylic orthiomethacrylic type acid is of the following general formula CH₂═C(R)--(C═O)--S--CH₂ --(CH₂)_(n) --OH wherein R is hydrogen or methyland n is as defined above.

The following examples present typical syntheses as described by thereaction schemes set forth above. These examples are understood to beillustrative only and are not intended to limit the scope of theinvention.

EXAMPLE 1 Preparation of S-(2-hydroxyethyl) thiomethacrylate

Silylation of 2-mercaptoethanol (37 mL, 0.5 mole) was performed withchlorotrimethylsilane (65 mL of 98%, 0.5 mole) using pyridine (0.5 mole)as a hydrogen chloride acceptor in a toluene solution. The mixture washeated to reflux and the solvent removed at atmospheric pressure. Aftersolvent removal, the crude product was vacuum distilled. Redistillationthrough a fractionating column gave a 54% yield of product (99+% purevia gas chromatographic analysis).

Lead (II) acetate trihydrate was slurried with toluene and heated toreflux to remove water of hydration. The theoretical amount of water wasobtained.

The hard crystalline lead acetate was slurried with the silylated2-mercaptoethanol and the mixture warmed to reflux. The yellow leadmercaptide was filtered and washed with ether. The dried salt wassuspended in ether and to this mixture was added methacryloyl chloride.From this ether solution was separated a white fluid liquid. Thisproduct had no sulfur odor and polymerized quite rapidly withtributylborane oxide (TBBO) initiator. Physical properties determined onthis material were: d²², 1.1441; N_(D) ²² 1.4975; M_(D) calc'd 37.43,found 37.06; solubility parameter, δ_(T) =24.9±0.7 (calculated using themethods of Hoy, Small, and Hoftyzer-Van Krevelin).

EXAMPLE 2 Preparation of S-(2-hydroxyethyl) thiomethacrylate

The starting material, trimethylsilyl 2-mercaptoethanol, was prepared byheating 2-mercaptoethanol with hexamethyldisilazane at 100° C. for 8hours with a slow sweep of nitrogen. The product was distilled through afour-plate Oldershaw column. The fraction boiling from 74° to 76.5° C.at water aspirator vacuum was collected. Gas chromatography of thisproduct indicated greater than 99% purity. The yield was 83.8% oftheory.

To the slurried lead (II) acetate trihydrate (76 g, 0.02 mole) in etherat ice bath temperature was added to 6.0 g, 0.04 moles of silylated2-mercaptoethanol. The hard, white crystalline acetate was transformedto a yellow amorphous precipitate. The mixture was stirred continuouslyat ice bath temperature for 1 hour. To this cold, stirred mixture wasadded 4.18 g. 0.04 moles of methacryloyl chloride. The yellow color wasdestroyed and a white gelatinous lead chloride precipitate was formed.This mixture was allowed to come to ambient temperature and stirred forseveral hours, after which time the lead chloride was removed byfiltration. To the clear filtrate was added 4-methoxyphenol as apolymerization inhibitor. The reaction mixture was passed through a 2.5cm by 18 cm activated alumina column. The column was then washed with150 mL of methylene chloride. The solvents were removed on a rotaryevaporator to yield a crude product of 4.1 g, or 70% of theory. Gaschromatography indicated mainly one compound. Analysis of the infraredspectrum showed that the silylated protective group had been hydrolyzedby the presence of acetic acid and water in the reaction mixture. Theproduct was free of sulfur odor.

The compounds of formula I have utility, including as hydrophilicsurface conditioners in biodental applications, such as bonding dentalrestoratives to dentinal structures. The compounds, when applied todentinal surfaces, wet & penetrate the surface and undergo in situpolymerization upon application of the dental adhesive and/or thecomposite resin. An inter-penetrating polymer network is thus formed tosecurely bind the resin to the dentinal structure. Notably, because ofthe presence of the sulfur atom in the compounds of the invention,scanning transmission electron microscopy, electron dispersivespectroscopy and auger spectroscopy can be used to localize thecompounds prior to or after polymerization and determine theirpenetration and wetting of the tissue. The ability to localize thecompounds without using radioactive labelling procedures is particularlyadvantageous because of the risks associated with the use of radioactivemarkers. The following example is illustrative of the use of thecompounds in this manner.

EXAMPLE 3 Use of HETMA in Dentinal Adhesive Systems

Ten-percent solutions of S-(2-hydroxyethyl) thiomethacrylate (HETMA) inacetone were applied to etched dentinal surfaces in the testing ofconventional dentinal adhesive systems which normally use HEMA as aconditioner. The HETMA penetrated and wet the demineralized dentinallayer well and was in intimate contact with the underlying dentin, asdetermined by scanning transmission electron microscopy and electrondispersive spectroscopy.

The invention is also directed to homopolymer and copolymer products ofthe S-(ω-hydroxyalkyl) esters of thioacrylic and thiomethacrylic acidsof formula I. The monomers of such esters can be polymerized using anyof various suitable polymerization techniques which are known in theart. In general, those polymerization techniques which are used tohomopolymerize and copolymerize HEMA are suitable for use with thecompounds of the present invention. In general, vinyl polymerization ofthe compounds of formula I proceeds at a faster rate in comparison toHEMA, making those compounds better suited for those applicationsrequiring a faster polymerization rate than is possible with HEMA.

The homopolymers and copolymers contain a residue of a compound offormula I and/or have a repeating unit of the following general formula:##STR11## wherein R and n are as defined above.

More specifically, the homopolymers of the invention comprise ahomopolymerization reaction product of a monomer of the formula:##STR12## wherein: R is hydrogen or methyl; and

n is 1, 2 or 3.

The copolymers comprise a reaction product of at least one monomer ofthe general formula I: ##STR13## wherein: R is hydrogen or methyl; and

n is 1, 2 or 3; and

at least one other monomer capable of polymerization with the monomer ofthe formula I.

In general, the compounds of formula I are capable of copolymerizingwith those comonomers that are capable of copolymerizing with HEMA. Suchcomonomers are well known and include alkyl acrylates, alkylmethacrylates, hydroxyalkyl acrylates and hydroxyalkyl methacrylatesamong others. For example, when a crosslinked polymer capable of forminga hydrogel is desired, the comonomer can be one or more of di(ethyleneglycol) diacrylate, di(ethylene glycol) dimethacrylate, tri(ethyleneglycol) diacrylate, tri(ethylene glycol) dimethacrylate, tetra(ethyleneglycol) diacrylate and tetra(ethylene glycol) dimethacrylate. If anoptically transparent copolymer is desired, particularly suitedcomonomers include methylmethacrylate and ethylmethacrylate.

The compounds of formula I can also be copolymerized with ionogeniccomonomers which include acids such as methacrylic, acrylic, andacrylate/methacrylate monomers containing phosphonic and sulfonic acidfunctionalities. An example of such a comonomer is 2-(sulfoxy) ethylmethacrylate. Other ionogenic comonomers include cationic comonomerssuch as 2-(N,N-diethylamino)ethyl methacrylate and2-(N,N-dimethylamino)ethyl methacrylate. Ampholytic terpolymers such asHETMA in combination with acrylic acid, 2-(N,N-diethylamino) ethylmethacrylate or 2-(N,N-dimethylamino)ethyl methacrylate are also withinthe scope of the invention.

In dental applications, the compounds of formula I can be copolymerizedwith bis-GMA (2,2'-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)]phenylpropane) and urethane dimethacrylates such as those obtained from thereaction of 1,6-hexane diisocyanate and HEMA or HETMA and the reactionof toluene 2,4'-diisocyanate and HEMA or HETMA. Reaction products ofanhydrides and HEMA or HETMA, for example the reaction product ofpyromellitic anhydride, trimellitic anhydride or succinic anhydride withHEMA or HETMA, can be used as a comonomer with HETMA or as a copolymerproduct.

The following examples illustrate methods of polymerization of thecompounds of formula I.

EXAMPLE 4 Homopolymerization of S-(2-hydroxyethyl) thiomethacrylateusing tri-n-butyl borane oxide as initiator

Five parts of S-(2-hydroxyethyl) thiomethacrylate (HETMA), whichcontained about 500 ppm of 4-methoxyphenol as an inhibitor, were mixedat 25° C. with one part of tri-n-butyl borane oxide (TBBO)(AMALGAMBONDCatalyst Parkell Biomedical Division). After about 10 seconds, there wasa slight exotherm and the reaction temperature rose to 27° C. Thetemperature was maintained at this level for 20 minutes. Then additionalTBBO was added to bring the reactant/initiator ratio to 2.6 parts HETMAto 1.0 parts TBBO. The polymerization mixture reached the stringpoint/cloud point after a total elapsed time of 30 minutes. After anadditional 5 minutes, the reaction mass was an elastomeric gel. An IRspectrum of the homopolymer after 80 minutes of reaction revealed thatthe absorption band at 1630 cm⁻¹ (vinyl unsaturation) in HETMA is notpresent in the homopolymer.

In a second preparation, four parts of HETMA (containing about 500 ppmof methoxyhydroquinone as an inhibitor) were mixed at 39° C. with onepart of TBBO. A rapid exotherm to 49° C. was noted during the first 30seconds. The mixture was heated to 55° C. and maintained at thistemperature. The string point/cloud point was reached after a totalelapsed time of 7.5 minutes, and the reaction mass became an elastomericgel after an additional 1 to 2 minutes.

EXAMPLE 5 Homopolymerization of S-(2-hydroxyethyl) thiomethacrylateusing 2,2'-azobis(2-methylbutyronitrile) as initiator

Twenty-five parts of HETMA (containing about 500 ppm of 4-methoxyphenolas an inhibitor) were mixed at 58° to 59° C. with one part of2,2'-azobis(2-methylbutyronitrile) (VAZO 67, Du Pont). An exotherm to61° C. was observed during the first 30 to 40 seconds. The temperaturewas maintained at 60° C. for 25 minutes. Then additional2,2'-azobis(2-methylbutyronitrile) was added to bring thereactant/initiator ratio to 12.4 parts HETMA to 1.0 parts2,2'-azobis(2-methylbutyronitrile). The polymerization mixture reachedthe string point/cloud point after a total elapsed time of 90 minutes at60° C. After an additional 10 minutes, the reaction mass was a soft gel.The reaction mass was a stiff, elastomeric gel after a total elapsedtime of 2.5 hours.

The polymers formed by the homopolymerization or copolymerization of thecompounds of formula I can be used in a variety of applications,including as diagnostic devices such as catheters, electrode catheters,carriers for enzyme immunoassay, gel-entrapped enzyme electrode probes,cell culture substrates, and electrophoresis gels, therapeuticapplications such as absorbent coatings for blood perfusion,hemodialysis membranes, blood oxygenators, degradable therapeuticsystems, drug-delivery systems, and medicated and soft contact lenses;implants such as intraocular lenses, artificial corneas, soft tissuesubstitutes, burn dressings, transdermal drug-delivery patches. Otheruses of the polymers can include as separation membranes. The polymerscan be formed into these useful products using suitable techniques knownto those of skill in the art.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects hereinabove set forthtogether with other inherent advantages.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth is to be interpreted as illustrative and not in alimiting sense.

Having thus described the invention, what is claimed is:
 1. A compoundof the general formula: ##STR14## wherein: R is hydrogen or methyl; andnis 1, 2 or
 3. 2. The compound as set forth in claim 1, wherein R ishydrogen or methyl and n is 1 or
 2. 3. The compound as set forth inclaim 1, wherein R is methyl and n is
 1. 4. The compound as set forth inclaim 1, wherein the compound is S-(2-hydroxyethyl) thiomethacrylate,S-(2-hydroxyethyl) thioacrylate, S-(3-hydroxy-n-propyl)thiomethacrylate, S-(3-hydroxy-n-propyl) thioacrylate,S-(4-hydroxy-n-butyl) thiomethacrylate or S-(4-hydroxy-n-butyl)thioacrylate.
 5. The compound as set forth in claim 1, wherein thecompound is S-(2-hydroxyethyl) thiomethacrylate or S-(2-hydroxyethyl)thioacrylate.